Method of producing dense,sintered bodies of uo2 or uo2-puo2 mixtures



United States 3,375,306 Patented Mar. 26, 1968 doc 3,375,306 METHOD OFPRODUCING DENSE, SINTERED BODIES F U0 OR U0 -Pu0 MlXTURES Lewis EricRussell, Oxford, John David Lawrence Harrison, Chilton, and Norman HarryBrett, Hathersage, near Sheffield, England, assignors to United KingdomAtomic Energy Authority, London, England No Drawing. Continuation ofapplication Ser. No. 106,185, Apr. 28, 1961. This application Jan. 11,1965, Ser. No. 424,822 Claims priority, application Great Britain, Apr.29, 1960, 15,240/60 5 Claims. (Cl. 264-.5)

ABSTRACT DF THE DISCLOSURE A method of producing dense sintered bodiesof either uranium dioxide or a mixture of uranium dioxide and plutoniumdioxide comprising pressing a powder to form a compact, heating thecompact at a temperature of from 1300 C. to 1600 C. in a sinteringatmosphere which is either carbon dioxide or carbon dioxide containingcarbon monoxide and cooling the sintered body in a reducing atmospherewhich, in the case where the body is uranium dioxide, is dry hydrogen,wet hydrogen or a mixture of carbon dioxide and carbon monoxide and, inthe case where the body is mixed uranium and plutonium dioxides, is wetsteam or carbon dioxide mixed with carbon monoxide. In this way, asintered body having a ratio of oxygen to metal atoms from 1.98:1 to2.02:1 and a density in excess of 95% of the theoretical density isobtained.

This application is a continuation of our copending application Ser. No.106,185, filed Apr. 28, 1961, and now abandoned, and the inventionrelates to the production of sintered uranium dioxide, and sinteredmixtures thereof with plutonium dioxide.

It has been discovered that uranium dioxide and mixtures thereof withplutonium dioxide may be sintered to very high densities by heating to atemperature of 1300 C. or above in an atmosphere of carbon dioxide, andthat in the case of mixtures containing from to plutonium dioxideparticularly high densities can be obtained by heating at suchtemperatures in a mixture of carbon dioxide and carbon monoxide.

According to the present invention, a method of producing a densesintered body consisting of uranium dioxide or a mixture thereof withplutonium dioxide comprises heating a compacted body of uranium dioxideor a mixture thereof with plutonium dioxide to a temperature of at least1300 C. in an atmosphere consisting of carbon dioxide or a mixturethereof with carbon monoxide.

Also according to the invention, a method of producing a dense sinteredbody consisting of a mixture of 10% to 30% plutonium dioxide andcorrespondingly 90% to 70% uranium dioxide comprises heating a compactedbody consisting of the said mixture to a temperature of at least 1300 C.in an atmosphere consisting of a mixture of carbon dioxide and carbonmonoxide in which the ratio of carbon monoxide to carbon dioxide is inthe range from 1:100 to 100.1.

The optimum ratio of carbon monoxide to carbon dioxide in order to givethe highest density depends on the proportion of plutonium dioxide inthe mixture, but is within the range from 1:100 to 1:10 for 10%plutonium dioxide, and within the range from 1:20 to 1:1 for 30%plutonium dioxide, the optimum ratios for plutonium dioxide proportionsbetween 10% and 30% being intermediate in range.

The optimum temperature for sintering in pure carbon dioxide varies withthe period at the sintering temperature. Maximum densities are attainedby sintering for 3 to 6 hours at 1300 to 1550 C., the density fallingoff slightly thereafter. At 1600 C., or above however, the same maximumdensity is attained within a few minutes but then falls off ratherrapidly.

The sintering temperature in mixtures of carbon monoxide and dioxide isnot so critical, but is preferably in the range 1400 C. to 1600 C.

A stoichiometric oxide product, that is, a product in which the ratio ofoxygen atoms to metal atoms is substantially 2.00: 1, may be produced bycooling the sintered material in a reducing atmosphere. Such anatmosphere may consist of pure hydrogen in the case of pure uraniumdioxide, but for mixtures of uranium dioxide and plutonium dioxidehydrogen containing a partial pressure of steam, or a mixture of carbondioxide and carbon monoxide, is preferred, since dry hydrogen reducessome of the plutonium to the trivalent state and thus lowers theoxygen/metal ratio below 2:1. Uranium is not reduced below thetetravalent state by hydrogen.

The ratio of carbon monoxide to carbon dioxide during cooling should bewithin the range from 1:10 to :1, the upper limit of monoxide being setby the decomposition of carbon monoxide to give carbon which occurs athigh temperatures when the carbon dioxide content of the mixture is lessthan about 1%. If the ratio of carbon monoxide to carbon dioxide is lessthan 1:10, then the oxygen/metal ratio will rise substantially higherthan 2.00: 1.

In specifying an oxygen/metal ratio of substantially 2.00:1 a ratiowithin the range from 1.98:1 to 2.02:1 is contemplated.

It is possible to select a ratio of carbon monoxide to carbon dioxidesuch that sintering and cooling can be carried out in the sameatmosphere to achieve both the highest possible density and anoxygen/metal ratio of substantially 2.00:1. Alternatively, the ratio maybe changed between siutering and cooling.

One advantage of the method of this invention is that the necessity forheating at temperatures above 1600 C., such as are necessary forsintering in an inert atmosphere or in hydrogen, can be avoided.

A further advantage is that it avoids the need for heating the compactslowly in the range 100 to 400 C., which is necessary for binder removalin air before sintering in an inert atmosphere or hydrogen, as describedin United States Patent No. 3,194,852.

The invention is of particular value for producing dense sintered bodiescontaining from 10% to 30% plutonium dioxide, such as are suitable foruse in a fast neutron nuclear reactor. Sintering of compacts containing10% to 30% plutonium dioxide in an atmosphere consisting of a mixture ofcarbon dioxide and carbon monoxide produces bodies which are not of highdensity, but are substantially chemically homogeneous as regards uraniumand plutonium, even if the original compacts consisted of discreteparticles of uranium dioxide and plutonium dioxide.

The nature of the invention, and the methods by which it is to beperformed, will become more apparent from the following examples:

EXAMPLE 1 50 gm. plutonium dioxide powder and 450 gm. uranium dioxidepowder were milled together in a polythene bottle containing porcelainballs for 16 hours at 100 rpm. Then 50 ml. of a 60% (by weight) solutionof polybutyl methacrylate in toluene were added as binder and themixture was passed through a sieve to give free-flowing granules. Afterdrying, the granules were pressed into pellets 0.375 inch in diameterand weighing 4.5 gm. each by double-end pressing at 30 tons per sq. in.The green density of the pellets was 6.3 gm./cc. The pellets were thenheated at the rate of 400 C. per hour to 1500 C. in an atmosphere ofpure carbon dioxide and kept at 1500 C. for 4 hours. After cooling inthe same atmosphere, the pellets were found to have a sintered densityof 10.6 gm./cc., which is 96% of the theoretical density of ahomogeneous mixture of 10% PuO and 90% U The sintered material was foundto consist of a solid solution of plutonium and uranium oxides, with anoxygen/ metal ratio of 2.14 to 1.

EXAMPLE 2 Pellets containing 1% plutonium dioxide and 99% uraniumdioxide were produced by a similar process to that described in Example1, but with a maximum temperature in carbon dioxide of 1400 C. Thedensity of the sintered pellets was found to be 10.6 gm./cc., which is96.5% of theoretical. In the sintered material, however, only 25% of theplutonium oxide was in solid solution with the uranium oxide.

EXAMPLE 3 Pellets consisting entirely of uranium dioxide were producedby a similar process to that described in Example 1, but after sinteringin carbon dioxide to 1550" C. were cooled in hydrogen. The density ofthe sintered pellets was 10.6 gm./cc., which is 96.5% theoretical, andthe oxygen/metal ratio of the product was 2.00 to 1.

EXAMPLE 4 Pellets consisting of plutonium dioxide and 90% uraniumdioxide were produced as described in Example 1, but after sintering incarbon dioxide'with a maximum temperature of 1550 C. for 4 hours werecooled in dry hydrogen to 950 and kept at this temperature for 4 hoursbefore cooling to room temperature in dry hydrogen. The dry hydrogencontained less than 1 part in 10,000 of water vapour. The sinteredpellets had a density of 10.5 gm./cc. which is 95% of theoretical,consisted of a solid solution of plutonium and uranium oxides, and hadan oxygen/metal ratio of 1.985 to 1.

EXAMPLE 6 Pellets consisting of 10% plutonium dioxide and 90% uraniumdioxide were produced as described in Example 5, except that the dryhydrogen was replaced by hydrogen which .had been saturated with watervapour at room temperature, i.e., which contained about 1% water vapour.The sintered pellets had a density of 10.6 gm./cc., which is 96% oftheoretical, consisted of a solid solution of plutonium and uraniumoxides, and had an oxygen/ metal ratio of 2.00 to 1.

EXAMPLES 7 TO 11 Pellets consisting of 10% plutonium dioxide and 90%uranium dioxide were prepared as in Example 5, except that the dryhydrogen was replaced by mixtures of carbon dioxide and carbon monoxidecontaining varying proportions of carbon monoxide, as shown in thefollowing table, which also shows the densities of the sintered pelletsand the oxygen/metal ratio for the products of each I example.

Density of product Ex. CO/COZ Oxygen/metal during cooling GllL/CC.Percent ratio in. Theoretical product A substantially stoichiometricoxygen/metal ratio is attained with a CO/CO ratio between 1:10 and100:1. The products of Examples 7 to 11 all consisted of solid.

solutions of plutonium and uranium oxides.

EXAMPLES 1214 C 0/0 02 ratio Density of product Oxygen/metal Ex. dtu'ingratio in sintering Gm.lce. Percent product theoretical The maximumdensity for pellets containing 10% plutonium dioxide is achieved with aratio of carbon monoxide to carbon dioxide in the range 1:100 to 1:10.The products of Examples 12 to 14 all consisted of solid solutions ofplutonium and uranium oxides.

EXAMPLE 15 EXAMPLES 16-20 Pellets consisting of 30% plutonium dioxideand 70% uranium dioxide were produced by a similar process to thatdescribed in Example 1, but were sintered at 1500 C. for 4 hours andthen cooled, in atmospheres consisting of pure carbon dioxide, ormixtures of carbon dioxide and carbon monoxide in which the ratios ofmonoxide to dioxide were as shown in the following table. The table alsoshows the densities of the sintered pellets and the oxygen/ metal ratiosfor the products of each example.

(JO/CO ratio Density oiproduct Oxygen/metal Ex. sinterlng ratio in andcooling Gm./cc. Percent product I theoretical 16 Pure CO1 10. 4 92. 5 2.14 17 1:100 10.5 94 2.12 18 1:20 10. 6 2. 04 19 1:10 10. 7 96.5 2. 02 201:1 10. (i5 96 2. 00

The maximum density for pellets containing 30% plutonium dioxide isachieved with a ratio of carbon monoxide to carbon dioxide in the range1 :20 to 1:1, and that the combinationof maximum densityandstoichiometric oxygen/metal ratio is achieved by 'sintering and coolingwith a ratio of carbon monoxide to carbon dioxide in the range 1:10to1:1. The products of Examples 16 to 20 allconsisted of solid solutionsof plutonium and uranium oxides.

We claim:

1. A method of producing dense, sintered bodies selected from the groupconsisting of bodies of uranium dioxide and a mixture of uranium dioxideand plutonium dioxide, said method comprising the steps of pressingparticulate material selected from the group consisting of uraniumdioxide and mixtures of uranium dioxide and plutonium dioxide to form acompacted body of said material; heating said compacted body to atemperature of from 1300 C. to 1600 C. in a sintering atmosphereconsisting essentially of an atmosphere selected from the groupconsisting of carbon dioxide, and carbon dioxide in admixture withcarbon monoxide, to sinter said body at said temperature; and coolingthe sintered body in a reducing atmosphere consisting essentially of anatmosphere selected from the group consisting of dry hydrogen, hydrogencontaining a partial pressure of steam, and carbon dioxide in admixturewith carbon monoxide when said body is uranium dioxide, and selectedfrom the group consisting of hydrogen containing a partial pressure ofsteam and carbon dioxide in admixture with carbon monoxide when saidbody is a mixture of uranium dioxide and plutonium dioxide; said cooledsintered body having a ratio of oxygen to metal atoms of from 1.98:1 to2.02:1 and a density of at least 10.5 g./cm.

2. A method according to claim 1 wherein said material is uraniumdioxide and wherein said sintering atmosphere is carbon monoxide andcarbon dioxide in admixture, the ratio of carbon monoxide to carbondioxide being from 1:100 to 100:1.

3. A method according to claim 2 wherein the reducing atmosphere iscarbon dioxide and carbon monoxide 6 in admixture, the ratio of carbonmonoxide to carbon dioxide being from 1:10 to 100/1.

4. A method according to claim 1 wherein said material is a mixture ofuranium dioxide and plutonium dioxide, said plutonium dioxide beingpresent in an amount of from 10 to 30% of said mixture, and wherein saidsintering atmosphere is carbon dioxide and carbon monoxide in admixture,the ratio of carbon monoxide to carbon dioxide being from 1:100 to 1:1.

5. A method according to claim 4 wherein the reducing atmosphere iscarbon dioxide and carbon monoxide in admixture, the ratio of carbonmonoxide to carbon dioxide being from 1:10 to 100/1.

References Cited UNITED STATES PATENTS 2,906,598 9/1959 Googin 233553,051,566 8/1962 Schwartz 264.5 3,063,793 11/1962 Rawson et al.252-301.1 3,081,249 3/1962 Whittemore 264.5 3,087,876 4/1963 Hendersonet al. 264-.5 3,141,782 7/1964 Livey et a1 264-.5 3,194,852 7/1965 Lloydet al. 264.5

OTHER REFERENCES AEC Documents: (1) HW-60276, April 1959, page 8; (2)TID-7546, BK2, November 1957, pages 420, 422, 434-436, 471 and 474-477.

Proceedings of Second U.N. Int., Conii, vol. 6, 1958, pages 599, 600,609, 610, 617-619 and 626-628.

L. DEWAYNE RUTLEDGE, Primary Examiner.

